Abstract
An effective solution to global human zinc (Zn) deficiency is Zn biofortification of staple food crops, which has been hindered by the low available Zn in calcareous soils worldwide. Many culturable soil microbes have been reported to increase Zn availability in the laboratory, while the status of these microbes in fields and whether there are unculturable Zn-mobilizing microbes remain unexplored. Here, we use the culture-independent metagenomic sequencing to investigate the rhizosphere microbiome of three high-Zn (HZn) and three low-Zn (LZn) wheat cultivars in a field experiment with calcareous soils. The average grain Zn concentration of HZn was higher than the Zn biofortification target 40 mg kg–1, while that of LZn was lower than 40 mg kg–1. Metagenomic sequencing and analysis showed large microbiome difference between wheat rhizosphere and bulk soil but small difference between HZn and LZn. Most of the rhizosphere-enriched microbes in HZn and LZn were in common, including many of the previously reported soil Zn-mobilizing microbes. Notably, 30 of the 32 rhizosphere-enriched species exhibiting different abundances between HZn and LZn possess the functional genes involved in soil Zn mobilization, especially the synthesis and exudation of organic acids and siderophores. Most of the abundant potential Zn-mobilizing species were positively correlated with grain Zn concentration and formed a module with strong interspecies relations in the co-occurrence network of abundant rhizosphere-enriched microbes. The potential Zn-mobilizing species, especially Massilia and Pseudomonas, may contribute to the cultivars’ variation in grain Zn concentration, and they deserve further investigation in future studies on Zn biofortification.
Highlights
Around 20% of the world population are suffering from zinc (Zn) deficiency, and the situation will become worse with the increase of atmosphere carbon dioxide (Myers et al, 2014; Smith and Myers, 2018)
Of the rhizosphere-enriched microbes exhibiting different abundances among cultivars, we identify 30 potential soil Zn-mobilizing bacteria that may contribute to the variation of grain Zn concentration among wheat cultivars
The six wheat cultivars exhibited significant differences in grain Zn concentration and shoot Zn uptake when grown on calcareous soils (ANOVA, P < 0.05, Figures 1B,C), consistent with the previous studies on these cultivars (Wang et al, 2018)
Summary
Around 20% of the world population are suffering from zinc (Zn) deficiency, and the situation will become worse with the increase of atmosphere carbon dioxide (Myers et al, 2014; Smith and Myers, 2018). An effective solution to address human Zn deficiency is to increase the grain Zn concentration of staple food crops like wheat, namely, Zn biofortification. Due to the poor mobility of Zn in soil, the absorption of Zn by plant roots mainly occurs in the rhizosphere, where the activities of roots and microorganisms can somewhat increase the amount of available Zn (Rehman et al, 2018). Inoculation of plant growth-promoting rhizobacteria (PGPR) like Bacillus, Arthrobacter, and arbuscular mycorrhizal fungi (AMF) has been reported to increase soil available Zn and crop grain Zn concentration, especially in soils with low available Zn (Singh et al, 2018; Coccina et al, 2019). Despite many reports on the isolation and functional verification of soil Znmobilizing bacteria, the roles of soil microbes in improving soil Zn availability and promoting crop Zn biofortification remain to be extensively explored
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